What is Ajax

The main challenge of web developer is to provide a simple, fast and attractive presentation to end users. These challenges can be achieved by using the powerful concept called AJAX.
The term Ajax, stands for "Asynchronous JavaScript and XML"
Below is the description for these three terms:

Asynchronous

When we send a request to server, we need to wait for the response to come back, We are free to do other things until we get the response. Probably the response may not come back immediately. So we need to set up a function that will wait for the response to be sent back by the server, and react to it once that happens.

JavaScript

Javascript is technology independent. We can integrate Javascript in multiple technologies. JavaScript is used to make/send a request to the server. Once the response is returned by the server, we will generally use some more JavaScript to modify the current page's DOM source in some way to show the user that the submission went through successfully.

XML

The data that which we receive back from the server will often be packaged up as a snippet of XML, so that it can be easily processed with JavaScript. This data can be anything you want, and as long as you want.

Ajax is technology independent. It can be implemented in JAVA, PHP, Ruby, Microsoft .NET and many more.